user
Explain the meaning of the idiomatic expression "rule of thumb".


assistant
The term "rule of thumb" is used to refer to guidelines or advice that is based on experience and common sense rather than exact measures. It originates from the 16th century English belief that an ordinary man was not able to judge the circumstances of a case any better than he could span it with his thumb. This phrase can be used to denote an approximate way of doing something instead of something exact.